We peer intently at particle counts. We stress over upward trends. We insist on compliance to gowning procedure and to appropriate cleanroom protocol on the part of employees. Of course, maintaining low levels of particles is a necessity. So is employee training or, as we prefer to call it, employee education. However, we cannot ignore other sources of contamination. Improperly maintained process equipment can compromise yield and add to both particulate and non-particulate contamination. In discussing process equipment used either for fabrication or for critical cleaning, we have to keep in mind that an understanding of the interaction of the chemical and the equipment as used under process conditions is essential for effective maintenance.
Routine maintenance
It should go without saying that routine maintenance of equipment is essential. So, we will say it anyway. There must be a written maintenance protocol; and there has to be a person in charge to assure that this protocol is carried out. Written documentation of findings and corrective action is a must. It is also important to specify the correct configuration of equipment in exquisite detail. Where equipment is modularized, it can be all too tempting to remove a rinse bath, or to repurpose a wash bath as a rinse tank. The cleaning agent should itself be specified; part of maintenance should include verifying that the correct cleaning agent at the correct concentration has been used.
Part of this routine maintenance ought to include visual observations. For example, in cleaning equipment, fixturing can be a major source of contamination. Over time, repeated exposure of fixtures to cleaning chemicals and other process chemicals, especially exposure over time to high temperatures and strong forces, can result in surface modification that includes shedding of particles. These particles can become airborne; and, of more immediate concern, they can contaminate the process bath and settle on the surface of the product. Your equipment maintenance checklist should include visual observation of fixtures.
Improper temperature settings and efficacy of the process force should be reviewed. Ultrasonics should be tested using the aluminum foil test or an appropriate probe.1,2 If the bath is operated at an elevated temperature, determine that the appropriate temperature has been achieved. This means that the appropriate temperature must be set and adhered to. The situation is a bit like Goldilocks—not too hot, not too cold, but just right. Roughly speaking, for many processes the reaction rate doubles with the temperature. If the temperature is too low, there will be sub-optimal cleaning. If the temperature is too high, the product may be compromised. If ultrasonics are used, there is an optimum temperature; cleaning effectiveness falls off on either side. If the ultrasonic tank is not temperature controlled, self-heating from the acoustic energy will cause the temperature to rise through the work day. The optimum temperature may be exceeded, leading to ineffective cavitation.
Where a spray system is used, nozzles need to be checked periodically. With line-of-site cleaning, it is surprising how often we find that component contamination can be decreased simply by making sure the nozzles are pointed in the correct direction and that they have not become clogged with chemicals. The problem could be avoided by careful, ongoing maintenance.
Speaking of clogged nozzles, process chemicals can themselves be a source of contamination. Equipment monitoring should include visual observations of the impact of process chemicals. For example, where aqueous blends are used, too often we observe deposits of cleaning agent additives on the surfaces of process baths. This situation should be avoided to assure good process consistency. Mold release agents may be used in an overly-enthusiastic manner and residue can build up on process equipment. It really takes a practiced eye to look over the equipment. People on the production line cannot be expected to do this, because they may not notice changes that occur gradually over time. The situation is analogous to assuming that high levels of an airborne chemical contaminant can be detected by odor alone; people who are in contact with the chemical day in and day out may lose sensitivity to the odor. The odor may be readily apparent to an independent observer, either from within or outside of the company.
Not all observations should be made visually. For example, if the stabilizers in chlorinated solvents have not been maintained, acidity formation may lead to visible corrosion of the metal. By the time damage is observed, it may be too late. We suggest that part of equipment maintenance include records of chemical maintenance and bath monitoring.
Final assembly
Those of you who are concerned only with final assembly in a cleanroom may be thinking that you do not have to be concerned about equipment maintenance because most processing occurs prior to cleanroom activities. Please step away from that thought. Go through the cleanroom to see if any processing occurs. No? Are you sure? Perhaps there are dispensers, often containing isopropyl alcohol, ethyl alcohol, acetone, or some other organic chemical. These are often used for final cleaning because they evaporate rapidly and leave little to no residue. In this instance, there is process equipment. If not properly documented and controlled, consumables such as gloves, swabs, and wipes can change a critical cleaning process to a critical contamination process. The sources of consumables have to be well-documented; and it may be appropriate to maintain certification for a given batch or lot.
Dispensers have to be monitored and replaced on a regular basis. Even the appropriate chemical-resistant container cannot be used indefinitely. There is likely to be contamination buildup over time as well as some leaching of materials of construction into the cleaning agent. Here is where the situation becomes a bit sensitive. Because many critical cleaning processes have so many variables, operators may develop an extremely close attachment to the dispenser they use for benchtop cleaning. The closest analogy we can make is that of a sport figure who wears their lucky hat or lucky socks to assure they win the game. While we are empathic to the desire for stability and consistency in manufacturing situations, the use by experienced technicians of a decades-old isopropyl alcohol dispenser (yes, we have observed this situation) is simply not good manufacturing practice. Particularly given such practices, a cleanroom equipment maintenance schedule must include an audit and inspection. If necessary, expunge ancient dispensers; wait until the operator goes on break if necessary. We have observed improvements in product yield and performance as a result or replacing old, “lucky” dispensers.
An ounce of prevention
Heed the adage from Ben Franklin. Counting particles is not enough. You have to control and maintain the equipment used in the manufacturing of the actual product. Equipment maintenance requires exquisite attention to detail. Developing a defined equipment maintenance audit can avoid process catastrophes. Even more important, defining the properties and parameters of the combination of chemicals and equipment helps to better define the process.
References
1. M. Hodnett, “Snap, Crackle, or Pop: How Do Bubbles Sound?,” Handbook for Critical Cleaning, Cleaning Agents and Systems, B. Kanegsberg and E. Kanegsberg, eds., CRC Press (2011), Chapter 16
2. L. Azar, “Principles and Quantitative Measurements of Cavitation,” ibid., Chapter 17.
Barbara Kanegsberg and Ed Kanegsberg (the Cleaning Lady and the Rocket Scientist) are independent consultants in critical and precision cleaning, surface preparation, and contamination control. They are the editors of The Handbook for Critical Cleaning, Second Ed., CRC Press. Contact: [email protected].
This article appeared in the March 2013 issue of Controlled Environments.
